WASHINGTON — A coalition of 12 states and several cities asked a federal appeals court Friday to make the Environmental Protection Agency reconsider its decision not to regulate heat-trapping greenhouse gases as air pollutants.

The case has big potential implications for numerous federal and state programs under the Clean Air Act, as well as for the auto industry. Along with other forms of transportation, motor vehicles account for about a third of all U.S. energy-related carbon dioxide emissions -- the chief gas scientists blame for global warming.

In a courtroom packed with auto industry representatives, environmentalists and government employees, three justices of the U.S. Court of Appeals for the District of Columbia Circuit sternly questioned lawyers for the states and the EPA. The judges wondered how far the government should go in the face of scientific uncertainty over global warming.

"We can't even tell what the weather's going to be two weeks from now, but these models tell us what the climate is going to be like 100 years from now," said Judge A. Raymond Randolph, whose questioning appeared to favor the EPA's position.

The judges did not indicate when they might rule in the case; such decisions typically take months.

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In August 2003, the EPA denied a petition from the nonprofit International Center for Technology Assessment and other groups that sought to impose new controls on auto emissions. The agency said it lacked authority from Congress to regulate greenhouse gases, based on a legal opinion from the agency's top lawyer -- who had reversed the Clinton-era legal opinion the gases should be regulated under the Clean Air Act.

Two months later, states and several cities formally challenged that decision. In the courtroom Friday, they argued that the EPA never adequately justified its decision.

"You don't have to look very far to find the authority that EPA claims it is missing," James R. Milkey, a Massachusetts assistant attorney general, told the judges Friday.

The states said the fact that another agency, the National Highway Traffic Safety Administration, regulates vehicles' fuel economy is beside the point. They also argued that Congress had included the word "climate" in the Clean Air Act for a reason.

"We don't need to guess what Congress meant on this," Milkey said. "The fuel economy issue is a red herring. ... The predictions of economic turmoil are not only completely overstated, they are irrelevant."

The EPA would have no easy way to regulate carbon dioxide from motor vehicles, said Jeffrey Clark, deputy assistant attorney general for the Justice Department's environment division.

"For CO2, there's no catalytic converter," he argued. "There's no catch mechanism. The only way to reduce them is through fuel economy. ... The point is it would usurp NHTSA's authority."

States challenging the EPA are California, Connecticut, Illinois, Maine, Massachusetts, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington, along with the U.S. territory of American Samoa and the cities Baltimore, New York and Washington, D.C.

They said the EPA acknowledged in testimony to Congress in 1998, 1999 and 2000 that the Clean Air Act gives the agency power to regulate pollution that causes global warming. Other states and cities also have tried to force federal regulation of greenhouse gases.